Microwave applicator for heating continuous web

ABSTRACT

A resonant cavity in the form of a right circular cylinder is excited in the TM010 mode by a source of microwave energy to generate an electric field with lines extending generally parallel to the axis of the cylindrical sidewall of the cavity. A pair of aligned slots are formed at diametrically opposite locations in the cavity sidewall for passing the web through the applicator in a plane which passes through the axis of the sidewall. The intensity of the electric field within the applicator increases in a radial direction of the cavity from a minimum at the sidewall to a maximum along the axis. Thus, the electric field, at its greatest intensity, extends parallel to the plane of the web for maximum coupling of energy to the web. Air is circulated through the applicator for carrying away moisture and for cooling the sidewalls of the cavity. An adjustable stub, mounted for radial movement in the cavity, tunes the resonant frequency of the cavity to match the frequency of the source.

United States Patent [721 lnventor Ray M. Johnson 118 Verde M158, Danville, Calif. 94526 [21 1 AppL No. 860,657 [22] Filed Sept. 24, 1969 [45] Patented Aug. 3, 1971 [54] MICROWAVE APPLICATOR FOR HEATING OTHER REFERENCES Radar Electronics Fundamentals, Navships 900.016 Washington D.C. June 1944 page 373 & 372

Primary Examiner--J. V. Truhe Assistant Examiner-L. H. Bender Attorneys-Carl C. Batz and Dawson, Tilton, Fallon &

Lungmus ABSTRACT: A resonant cavity in the form of a right circular cylinder is excited in the TM mode by a source of microwave energy to generate an electric field with lines extending generally parallel to the axis of the cylindrical sidewall of the cavity. A pair of aligned slots are formed at diametrically opposite locations in the cavity sidewall for passing the web through the applicator in a plane which passes through the axis of the sidewall. The intensity of the electric field within the applicator increases in a radial direction of the cavity from a minimum at the sidewall to a maximum along the axis. Thus, the electric field, at its greatest intensity, extends parallel to the plane of the web for maximum coupling of energy to the web. Air is circulated through the applicator for carrying away moisture and for cooling the sidewalls of the cavi ty. An adjustable stub, mounted for radial movement in the cavity, tunes the resonant frequency of the cavity to match the frequency of the source.

PATENTED AUG 3 l9?! WEB SOURCE REELER RAY M. JOHNSON MICROWAVE APPLICATOR FOR HEATING CONTINUOUS WEB BACKGROUND AND SUMMARY The present invention relates to a system for applying microwave energy to a web to heat it; more particularly, it pertains to a microwave applicator adapted to heating a web which is continuously fed through the applicator so that the heating may be carried on uninterrupted.

One of the early developments in the application of microwave energy to material was the so-called batch type oven in which food material is placed within a multimode cavity, the cavity sealed, and the oven excited by a source of microwave energy at a single frequency. Another development was a continuous type microwave ovenfor feeding microwave power into an elongated tunnel through a series of slit openings spaced longitudinally of the tunnel. These tunnels are also multimode cavities since a number of different modes are excited within the heating chamber.

Waveguides have been folded into a serpentine configuration to permit the product to pass through aligned slots in opposing broadwalls of a number of wave guide sections folded so that all of the slots are aligned. Energy is coupled into one end of the waveguide to excite a traveling wave which makes a number of passes through the material transverse of the direction of movement of the material finally terminating in a water load.

A copending, co-owned application of Ray M. Johnson for CONTINUOUS MICROWAVE HEATING OR COOKING SYSTEM, Ser. No. 816,722, filed Apr. 16, 1969, describes a rectangular waveguide applicator excited in the TE mode wherein the material being treated is conveyed to the applicator along the direction of power flow. The symbol TE" refers to the Transverse Electric field vector; and TM" refers to the Transverse Magnetic field vector. Reject filters are provided in this system at the input and exit apertures to prevent the escape of microwave energy; and a termination is provided downstream of the heating chamber to absorb microwave energy not absorbed in the material being treated.

In another copending, co-owned application of Ray M. Johnson and Frank J. Smith for RESONANT CAVITY MICROWAVE APPLICATOR, Ser. No. 852,374, filed Aug. 22, 1969 an applicator is disclosed for heating single-ended filament which is continuously fed along the axis of a resonant cavity.

In the present invention, a resonant cavity in the form of a right circular cylinder is excited in the TM mode by a source of microwave energy. The magnetic field vectors extend in concentric circles, and each circular magnetic. field line extends in a plane perpendicular to the axis of the applicator and has its center located on that axis. As used herein, the word cavity" refers to the conductive wall members as well as the volume defined thereby, which volume is sometimes referred to as the heating chamber. Thus, the resonant cavity provides a heating chamber for the web being treated.

When the cavity is thus excited, electric field lines are generated within the heating chamber extending generally parallel to the axis of the cylinder. The intensity of the electric field increases in a radial direction from a minimum at the sidewalls to a maximum along the axis; and the intensity is substantially uniform along any given line parallel to the axis. That is to say, the intensity profile of the electric field in a plane transverse of the axis is at a maximum in the center of the plane and then decreases as one proceeds radially toward the sidewall. Approximately the same intensity profile exists for each such plane along the axis of the cavity.

Slots for passing the web transversely through the axis of the cavity are formed on diametrically opposite sides of the cylindrical sidewall. The slots may communicate with each other by a third slot formed in one of the transverse end plates thus forming two cantilevered half-cylinders out of the cavity. This latter arrangement facilitates loadingof .a weblike material into the cavity and passing the same in a direction transverse of and through the axis of the cavity. Thus, the material being treated passes through the zone of maximum electric field intensity and is oriented such that the electric field lines are parallel with the web surface for maximum coupling of microwave coupling of microwave energy into the web.

Provision is made for forcing air into the interior of the cavity for carrying away moisture and for cooling the walls of the cavity which are heated by the electrical currents conducted by the interior surfaces of the walls.

An adjustable screw is located in the cylindrical sidewall to adjust the resonant frequency of the cavity; and tuning of the cavity to the source frequency under load is facilitated by means of a coupling probe located on one of the end plates. The tuning screw'is adjusted to tune the resonant frequency of the cavity to the operating frequency of the excitation source.

Since it is possible to support the TE mode of propagation in a cavity when the TM mode is propagated, the axial length of the cavity is preferably maintained less than one-half wavelength of the excitation frequency to confine the electric field vector to the TE mode.

A number of cavities could be arranged in tandem with the web being transported sequentially through all of the cavities, if needed. The cavities may be excited by a loop, a probe, a coupling aperture, or a feed waveguide.

Thus, the present invention provides a microwave applicator which is simple in construction and useful in applying highly concentrated microwave energy in an orientation which permits maximum coupling of microwave energy into a web being treated in a reasonably small space and with high efficiency, safety and reliability.

Other features and advantages of the instant invention will be apparent to persons skilled in the art from the following detailed description of alternative embodiments accompanied by the attached drawing.

THE DRAWING FIG. 1 is a perspective view of an applicator adapted to heat a web according to the present invention;

FIG. 2 is a transverse cross section view of the applicator of FIG. I;

FIG. 3 is a perspective view of an alternate embodiment of an applicator, partially cut away to show the interior; and

FIG. 4 is a transverse cross section view of the applicator of FIG. 3.

DETAILED DESCRIPTION Referring first to FIGS. 1 and 2, there is shown an applicator generally designated 10 which is adapted to apply microwave energy to a web denoted W. The web W is fed to the applicator from a source diagrammatically illustrated by the box 11; and after passing through the applicator, it may be collected by any suitable means (such as a reeler diagrammatically shown by the block 12) or transported through a second, independent applicator similar to the one shown.

The web material may be a woven or nonwoven, natural or synthetic fiber, or any material that is nonconductive. However, better results in heating the web if it has a relatively high dielectric loss factor. Further, the web shown may be a conveyor belt of low-loss material for supporting thin articles being heated, as long as the articles pass through the applicator without substantial loss of the microwave energy by radiation through the passage slot, as will be clear from subsequent description.

The heating applicator 10, as seen in perspective in FIG. 1, takes the form of a right circular cylinder including a cylindrical sidewall 13 and first and second transverse end plates 14 and 15 (FIG. 2) which define the heating chamber. The cylindrical sidewall 13 defines an axis which extends transverse of the direction of motion of the web W.

The transverse end plates 14 and 15 together with the cylindrical sidewall 13 provide the resonant electromagnetic cavity which has the form of a right circular cylinder. These boundary walls are sometimes collectively referred to herein as the. housing of the cavity. A slot 16 is formed in the housing of the cavity. The slot includes three communicating sections: one segment is designated 17 in FIG. 2 and extends along the sidewall 13 parallel to its axis; a second segment 18 is formed in the end plate 14; and the third segment 19 is formed in the sidewall 13 at a location diametrically opposite and parallel to the slot 17. Thus the slot 16 is formed in a plane which passes through the axis of the cavity; and it divides the housing of the cavity into cantilevered half sections.

By forming the slot 16 in the continuous fashion illustrated in FIGS. 1 and 2, an operator may first connect the web to the transport mechanism before loading it into the applicator. Loading is simply and safely accomplished by flexing the web laterally, aligning it with the segment 17, placing it in its operating position. If loading or unloading the web does not present a problem, the segment 18 in the end plate is not necessary.

A threaded aperture 21 is formed in the sidewall 13 for receiving a flange member 22. The flange 22 is adapted to be connected to a coaxial cable or waveguide for coupling a source of microwave energy (not shown) to the applicator to excite the resonant cavity. A feed center conductor 23 extends through the aperture 21 and is provided with an end coupling loop 24 for exciting the resonant cavity.

An externally threaded screw member 25 is also received in a threaded aperture located in the cylindrical sidewall 13. The screw 25 is adjustable in a radial direction in the sidewall for tuning the resonant frequency of the cavity to that of the source. A locknut 26 secures the tuning screw 25 in a fixed position once the adjustment is made.

A coupling probe generally designated 28 in FIG. 2 is threadably received in the end plate and it includes a center conductor 29 which partially extends within the cavity and is supported by means of a connector (which may be a Type N coaxial connector). The center connector 29 is then connected by means of coaxial cable to a power meter to monitoring the power coupled from the cavity. It will be observed from FIG. 2 that the location of the coupling probe 28 is toward the outer peripheral edges of the plate 15. This minimizes the amount of power coupled from the cavity (preferably, of the order of milliwatts). A second function of this probe is to permit matching the input impedance of the cavity to the output impedance of the source by the coupling loop by merely observing power levels.

Also secured to the sidewall 13 is a conduit 32 for communicating the heating chamber with a source of pressurized air to continually circulate air through the cavity. The air will be exhausted through the slot 16 described above; and this feature is important for applications in which the web (or an article it supports) contains water desired to be evaporated because the combined effect of the heated air and applied microwave energy is advantageous in reducing the transportation time and power requirements.

As already mentioned, by exciting the cavity in the TM mode, it is possible to generate the TE mode in which electric field vectors extend transverse of the axis of the cylindrical sidewall. The resulting currents would be interrupted by the slot 16 and thereby produce radiation of the energy in this mode. The TE mode may be suppressed by maintaining the axial length of the cavity less than one-half of the cavity wavelength for the TE. mode.

In the alternative embodiment shown in FIG. 3, the applicator heats a web designated W. The applicator, generally denoted 35, includes a cylindrical sidewall 36 and first and second transverse end plates 37 and 38. In this embodiment, the opening for admitting passage of the web takes the form of two separate slots 39 and 40 formed respectively in diametrically opposing locations of the sidewall 36.

The cavity provides a heating chamber 46; and a tuning screw 51 similar to the previously identified tuning screw 25 is threadably received in the cylindrical sidewall 36 at about its longitudinal center. Air input conduit for coupling a source of pressurized air to the interior of the cavity is designated 53. A power-monitoring probe 54 may be secured to the end plate 38. Such a probe may be similar to the probe 28 of the embodiment of FIGS. 1 and 2.

Whereas the cavity of the embodiment illustrated in FIGS. 1 and 2 was excited by means of a coupling loop, the embodiment of FIG. 3 is excited by means of an input wave guide which couples output energy from a microwave source, not shown. An adapter for connecting the waveguide to the cavity is designated generally by reference numeral 55, and it includes a mounting flange 56 and a waveguide tapered section 57 integral at its widest portion to the flange 56. An input coupling aperture 58 is formed in the cylindrical sidewall 36 of the cavity 45; and the narrow end of the tapered waveguide section 57 is secured to the exterior surface of the cylindrical sidewall 36 about the input coupling aperture 58. The side edges of the aperture 58 as at 59 and 60 are designed as matching iriss to match the characteristic impedance of the input feed waveguide to the input impedance of the cavity.

The resonant cavity may also be excited by means of an input probe coupled to a coaxial cable or, alternatively, the cavity may be symmetrically excited with coupling apertures properly phased so that the TE mode is not excited.

In operation, the cavity of the microwave applicators in each of the illustrated embodiments is excited in the TM mode. The magnetic field lines extend substantially circumferentially about the axis of the cavity in a series of concentric circles. That is, the magnetic field lines, in any one plane transverse of the axis, are arranged in concentric circles, all centered on the axis of the cavity. The intensity of the mag- I netic field is at a maximum near the interior cylindrical wall of the cavity; and it diminishes proceeding toward the axis of the cavity.

The electric field lines are orthogonal to the magnetic field lines; hence, the electric field lines extend perpendicularly between the transverse end plates in substantially parallel arrangement, all of the electric field lines being parallel to the axis of the cavity. The intensity of the electric field lines is uniform in the axial direction and varies along a radius as the Bessel function of the first kind, order zero. That is, in a transverse plane, the electric field intensity is at a minimum along the cylindrical sidewalls of the cavity and increases to a maximum along the axis of the cavity; and the profile of the electric field intensity is substantially the same for any such transverse plane taken along the axis. This is considered an important advantage of the present invention because it permits the treatment of webs of different width while insuring that the heat induced along an elemental area across the web will be uniform-thus overcoming one of the main disadvantages of multimode cavities.

The intensity also increases with frequency. Heating of the web, of course, increases with electric field intensity; but the intensity must be maintained at a safe level to avoid breakdown.

The radius of the cavity is selected to be slightly lower than the wavelength of the lowest expected resonant frequency of the particular power oscillator used to excite the cavity. By inserting the tuning screw into the cavity (or chamber) at a position of high magnetic field, the resonant frequency can be increased. This increase is a function of screw diameter and depth of insertion.

In addition to the mismatch caused by the introduction of the web, an additional frequency shift may be caused by the heating of the cavity due to PR losses which will cause an expansion of the walls of the cavity. Thus, the compensation afforded by adjustment of the tuning screws in each embodiment offsets such frequency shifts.

The condition of resonance permits an electric field to be generated intensity than would otherwise be possible (as for example, with multimode cavities), and these increased intensities may be attained with moderate input power. High electric field intensities can be directly related to rapid heating rates in a lossy dielectric material being treated.

The current flow exhibits the skin effect phenomenon along lines substantially parallel with the slots formed in the two illustrated embodiments for transportation of the web. Since the slots do not interrupt current, there is little or no escape of microwave energy through radiation.

The intensity requirement for the electric field will, of course, vary from application to application depending upon the axial length of the applicator and the load presented to the source. It will, however, be observed that the web passes transversely through the location of maximum field intensity (i.e. along the axis of the cylindrical sidewall) and that the orientation of the electric field lines is parallel with the surface of the material being treated at the location of maximum field intensity. This effects a maximum coupling of microwave energy into the material being treated for heating it, or in the case in which the web is a conveyor, for heating articles supported by it.

Impedance matching between the source and the resonant cavity in the device of FIGS. 1-2 is achieved by orientation of the end loop 24 of the center conductor 23 projecting into the cavity to couple the maximum amount of power into the cavity. In the embodiment of FIG. 3, impedance matching is provided by a properly designed iris as part of the coupling aperture 58. Matching of the frequency at which the cavity resonates to the source frequency in order to minimize the amount of power reflected back to the source is accomplished by means of the tuning screws. With the frequency of the cavity tuned to the frequency of the source and the impedance of the cavity matched to the impedance of the source and feeding waveguide, a maximum field intensity within the cavity and a higher efficiency are achieved.

The monopole coupling probes 28 and 54 located in the end plates of the cavities couple a small amount of power (of the order of milliwatts) from their respective cavities. For a given incident power, the cavity is tuned by means of the tuning screw until the power coupled out through the probe is maximized. This peaking of output power for a constant input power indicates the resonant condition of the cavity-that is, the frequency is adjusted until resonance is achieved. The tuning screw permits the exact matching of the cavity resonant frequency with the oscillation frequency of the magnetron (or other source of microwave energy).

The input power coupler places this loop in the side of the cylinder at a position of high magnetic field intensity but low electric field intensity. By avoiding input coupling (or excitation) at a position of high electric field intensity, the possibility of electric field breakdown at the probe position is significantly reduced.

The provision of the means for blowing air into the cavity insures that the air within the resonator does not become saturated with moisture; and it serves to cool the interior surface of the structure since the power dissipated in the cavity walls results in a pronounced heating ofthe metal.

For all other parameters constant, the efficiency of the resonator is strongly influenced by the resistivity of the metal used for fabrication (of particular importance because of the skin effect is the metal forming the interior boundary of the cavity). The lower the resistivity of this metal, the smaller are the losses to the resonator walls for a given electric field intensity along the centerline of the cavity. The power coupled to the material along the centerline is directly proportional to the square of the electric field intensity-thus, the resultant increase in efficiency is apparent.

Silver has a very low resistivity and therefore is a desirable material for use in interior surface of a cavity. The plating thickness of 0.2 mil is sufficient to exceed the skin depth of the currents at 2.45GHz. and thereby to provide the necessary shielding from the bulk portion of the metal comprising the resonator. The use of metals with an even greater conductivity will result in a corresqpnding increase in efficienc Having thus descri ed in detail alternative em odiments of the invention, persons skilled in the art will be able to substitute equivalent elements for those described to perform similar functions or to otherwise modify the structure or operation of these detailed showings departing from the inventive principle; and it is, therefore, intended that all such modifications and substitutions be covered as they are embraced within the spirit and scope of the invention.

Iclaim:

1. A system for applying heat to a web or to articles supported by a web comprising: a source of microwave energy, a resonant cavity having a cylindrical conductive sidewall and an axial length less than one-half wavelength of the resonant frequency thereof to suppress the TE mode, excitation means receiving microwave energy from said source to excite said cavity in substantially the TM mode, said cavity further including means for admitting transportation of said web transverse of the axis of said sidewall wherein the surface of said web extends substantially parallel to the electric field lines within said cavity.

2. The system of claim 1 further comprising conduit means for coupling a source of pressurized air into said cavity means to circulate air therethrough, whereby moist air is purged from the interior of said cavity and the sidewalls of said cavity are maintained in equilibrium temperature.

3. The system of claim 1 further comprising adjustable screw means threadedly received in the sidewall of said cavity and extending into the interior thereof to a predetermined length for varying the resonant frequency of said cavity to tune said resonant frequency of said cavity to the operating frequency of said source.

4. The system of claim 3 further comprising probe means extending partially within said cavity to couple power therefrom to monitor the power within said cavity and assist in adjusting the resonant frequency thereof to the frequency of said source.

5. The system of claim 1 wherein said cavity is further bounded by first and second end plates and wherein said means for admitting transportation of said web comprises a continuous slot formed in diametrically opposite locations of said sidewall and extending through one of said transverse end plates. 

1. A system for applying heat to a web or to articles supported by a web comprising: a source of microwave energy, a resonant cavity having a cylindrical conductive sidewall and an axial length less than one-half wavelength of the resonant frequency thereof to suppress the TE11 mode, excitation means receiving microwave energy from said source to excite said cavity in substantially the TM010 mode, said cavity further including means for admitting transportation of said web transverse of the axis of said sidewall wherein the surface of said web extends substantially parallel to the electric field lines within said cavity.
 2. The system of claim 1 further comprising conduit means for coupling a source of pressurized air into Said cavity means to circulate air therethrough, whereby moist air is purged from the interior of said cavity and the sidewalls of said cavity are maintained in equilibrium temperature.
 3. The system of claim 1 further comprising adjustable screw means threadedly received in the sidewall of said cavity and extending into the interior thereof to a predetermined length for varying the resonant frequency of said cavity to tune said resonant frequency of said cavity to the operating frequency of said source.
 4. The system of claim 3 further comprising probe means extending partially within said cavity to couple power therefrom to monitor the power within said cavity and assist in adjusting the resonant frequency thereof to the frequency of said source.
 5. The system of claim 1 wherein said cavity is further bounded by first and second end plates and wherein said means for admitting transportation of said web comprises a continuous slot formed in diametrically opposite locations of said sidewall and extending through one of said transverse end plates. 